The present invention relates to a method and apparatus for calibrating a moisture sensor and transmitter. More particularly, the present invention relates to an improved calibration apparatus and method which permits automatic recalibration of a moisture transmitter in the field without disconnecting the moisture transmitter from a process pipe in which the moisture transmitter is installed.
Moisture measuring systems are well known for sensing moisture in a process pipe. Typical systems include remote transmitters with microprocessor electronics, a sensor element mounted in a probe, a flow chamber, a weather-proof enclosure and assorted fittings. The moisture probe can be installed on to a process pipe in a variety of ways. For instance, the moisture sensor can be mounted in an in-line mounting in a pipe, mounted in a pipe expansion, mounted in an elbow for small pipe diameters, or mounted in a by-pass line to facilitate removal of the moisture probe.
One problem associated with moisture transmitters is that the sensors need to be recalibrated, at least every year. Conventional moisture probes must be detached from the process pipe and sent back to the manufacturer for recalibration. For instance, in a common installation, a moisture probe is mounted in a bypass flow cell with a valve at the outlet. The transmitter provides a display and a current output. When recalibration is needed, the moisture probe must be removed from the process pipe and sent back to the factory for recalibration. This causes a down time of at least 2-3 weeks. After return of the moisture probe, new calibration data must be entered into the transmitter by changing an EPROM or programming through manual entry keys.
Advantageously, the apparatus and method of the present invention are designed to permit field calibration of the moisture transmitter without removing the moisture transmitter from the process pipe. Using the field calibrator, the moisture transmitter can be recalibrated in about 24 hours without removing the moisture transmitter from the process pipe. The recalibration is done on-site and automatically. The user does not have to be knowledgeable on how to use a moisture analyzer or maintain it. Conventional recalibration methods impose on the user a duty to know how to match the recalibrated sensor to the transmitter by entering data or opening the transmitter for an EPROM change.
According to one aspect of the present invention, an apparatus is provided for calibrating a measuring transmitter which provides an output signal proportional to a parameter of interest measured by a sensor in the measuring transmitter without removing the measuring transmitter from its normal operating position. The apparatus includes a housing, a calibration sensor element located in the housing for sensing said parameter of interest, and a tube for coupling the housing to the measuring transmitter, thereby exposing the calibration sensor to the same parameter of interest as the measuring sensor in the measuring transmitter. The apparatus also includes a communication link for coupling the measuring sensor to the calibration sensor, and means for processing data related to the parameter of interest detected by the calibrating sensor, for comparing the processed data related to the parameter of interest detected by the calibrating sensor to processed data related to the parameter of interest detected by the measuring sensor, and for calibrating the measuring sensor based on the comparison.
In the illustrated embodiment, the apparatus further includes means for storing original calibration data related to the parameter of interest from the measuring transmitter, a maximum detected value of the parameter of interest from both the measuring sensor and the calibration sensor, and a minimum detected value of the parameter of interest for both the measuring sensor and the calibration sensor. The apparatus also includes a signal generator for generating a period having a variable length as a function of the parameter of interest being measured.
Also in the illustrated embodiment, the apparatus includes a clock for generating equally spaced pulses, and means for determining a count of pulses occurring within a period representing the maximum detected value of the parameter of interest for the calibration sensor, a period representing the maximum detected value of the parameter of interest from the measuring sensor, a period representing the minimum detected value of the parameter of interest for the calibration sensor, and a period representing the minimum detected value of the parameter of interest for the measuring sensor.
The processing means further calculates a difference between the original calibration data for the maximum detected value of the parameter of interest and the maximum detected value of the parameter of interest from the calibration sensor and a difference between the original calibration data for the minimum detected value of the parameter of interest and the minimum detected value of the parameter of interest from the calibration sensor. A display unit is coupled to the processing means to provide a visual indication of the progress of the calibrating means.
According to another aspect of the present invention, a method is provided for recalibrating a measuring transmitter which provides an output signal proportional to a parameter of interest measured by a measuring sensor in the measuring transmitter without removing the measuring transmitter from its normal operating position. The method includes the steps of providing a calibration sensor for sensing said parameter of interest, and connecting the calibration sensor to the measuring transmitter device, thereby exposing the calibration sensor to the same parameter of interest as the measuring sensor. The method also includes the steps of processing data related to the parameter of interest detected by the calibrating sensor, comparing the processed data related to the parameter of interest detected by the calibrating sensor to processed data related to the parameter of interest detected by the measuring sensor, and recalibrating the measuring sensor based on the comparison.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.